As known in the art, LEDs (which as used herein also encompasses organic LEDs, or OLEDs) are solid-state semiconductor devices that convert electrical energy into electromagnetic radiation that includes visible light (wavelengths of about 400 to 750 nm). An LED typically comprises a chip (die) of a semiconducting material doped with impurities to create a p-n junction. The LED chip is electrically connected to an anode and cathode, all of which are often mounted within a package. Lamps (bulbs) that utilize LED technology provide a variety of advantages over more traditional incandescent and fluorescent lamps, including but not limited to a longer life expectancy, high energy efficiency, and full brightness without requiring time to warm up. Because, in comparison to other lamps such as incandescent or fluorescent lamps, LEDs emit visible light that is more directional in a narrower beam, LED-based lamps have traditionally been utilized in applications such as automotive, display, safety/emergency, and directed area lighting. However, advances in LED technology have enabled high-efficiency LED-based lighting systems to find wider use in lighting applications that have traditionally employed other types of lighting sources, including omnidirectional lighting applications previously served by incandescent and fluorescent lamps. As a result, LEDs are increasingly being used for area lighting applications in residential, commercial and municipal settings.
FIG. 1 represents a nonlimiting commercial example of an LED-based lighting unit suitable for area lighting applications. The lighting unit (hereinafter, lamp) 10 is represented as a General Electric Energy Smart LED bulb or lamp (ANSI A19 type) configured to provide a nearly omnidirectional lighting capability. LED-based lighting units of various other configurations are also known. As represented in FIG. 1, the lamp 10 comprises a transparent or translucent cover or enclosure 12, an Edison-type threaded base connector 14, a housing or base 16 between the enclosure 12 and the connector 14, and heat-dissipating fins 18 that enhance radiative and convective heat transfer from the base 16 and enclosure 12 to the surrounding environment.
An LED-based light source, often an LED array comprising multiple LEDs, is typically located at the lower end of the enclosure 12 adjacent the base 16. Because LEDs emit visible light in narrow bands of wavelengths, for example, green, blue, red, etc., combinations of different LEDs are often combined in LED lamps to produce various light colors, including white light. The LEDs may be mounted on a carrier mounted to or within the base 16, and may be encapsulated on the carrier, for example, with a protective cover, often formed of an index-matching material to enhance the efficiency of visible light extraction from the LEDs. As a nonlimiting example, FIG. 2 represents a portion of an LED device 20 of a type that comprises a dome 22 that serves as an optically transparent or translucent envelope enclosing an LED chip 24 mounted on a printed circuit board (PCB) 26. A phosphor may also be used to emit light of color other than what is generated by an LED. For this purpose, the inner surface of the dome 22 may be provided with a coating 28 that contains a phosphor composition, in which case electromagnetic radiation (for example, blue visible light, ultraviolet (UV) radiation, or near-visible ultraviolet (NUV) radiation) emitted by the LED chip 24 can be absorbed by the phosphor composition, resulting in excitation of the phosphor composition to produce visible light that is emitted through the dome 22. As an alternative, the LED chip 24 may be encapsulated on the PCB 26 with a coating, and such a coating may optionally contain a phosphor composition for embodiments in which LED-phosphor integration with LED epitaxial (epi) wafer or die fabrication is desired.
To promote the capability of the lamp 10 to emit visible light in a nearly omnidirectional manner, the enclosure 12 is represented in FIG. 1 as substantially ellipsoidal or spheroidal in shape. To further promote a near omnidirectional lighting capability, the enclosure 12 can be formed of a material that enables the enclosure 12 to function as an optical diffuser. As a nonlimiting example, the enclosure 12 may be or may include an assembly comprising a pair of semi-elliptical or semispherical diffusers between which an internal reflector (not shown) may be disposed, such that visible light generated by the LED devices is directed into the interior of the enclosure 12, a portion of the generated light is reflected by the reflector into the diffuser nearer the base 16, through which the reflected light is distributed to the environment surrounding the lamp 10. The remainder of the generated light passes through an opening in the reflector and enters the second diffuser, through which the passed light is distributed to the environment surrounding the lamp 10. Materials commonly employed to produce the enclosure 12 include polyamides (nylon), polycarbonate (PC), polystyrene (PS), and polypropylene (PP) that typically contain a filler, for example, titania (TiO2) to promote refraction of the light and thereby achieve a white reflective appearance. The inner surface of the enclosure 12 may be provided with a coating (not shown), for example, a coating that contains a phosphor composition.
Area lighting applications typically require significantly higher electrical power levels for LED-based light units (such as of the type represented in FIG. 1) to produce greater amounts of light. A portion of the electrical power is converted into heat, which is preferably dissipated from the LED to promote the efficiency and reliability of the LEDs. While incandescent and fluorescent lamps typically dissipate a significant amount of heat, e.g., via radiation through the lens of the lamp, this approach has often been found to be inadequate for use in high power LED-based lighting units of types suitable for area lighting applications. Consequently, high power LED-based lighting units are often designed to dissipate heat via conduction by directly attaching the LED chip/package to a substrate capable of serving as a heat sink, and/or via convection and radiation with fins (e.g., 18 in FIG. 1) located externally of the LEDs. Nonlimiting examples of advanced fin and heat sink designs and materials are disclosed, respectively, in U.S. Patent Application Publication Nos. 2011/0169394 and 2011/0242817, each of which discloses the use of a polymer composite comprising a carbon nanotube filler in a polymer matrix. Various other thermal management techniques have also been proposed, such as active cooling techniques, nonlimiting examples of which are disclosed U.S. Patent Application Publication Nos. 2004/0190305 and 2012/0098425. While effective, thermal management systems can present a number of design challenges, particularly in view of the compact and lightweight designs typically desired for lighting units.